This invention relates to systems and methods of using of hydrocyclone cleaners for cleaning and thickening a suspension of papermakers' fibers, otherwise commonly known as papermakers' stock. The methods and systems include a forward hydrocyclone cleaning subsystem followed by a reverse hydrocyclone cleaning subsystem. In each cleaner a feed flow is divided into at least one accepts and one rejects stream.
The terms "forward cleaning" and "reverse cleaning" have become well understood in the art of cleaning papermakers' stock, and relate to the manner in which a cyclone-type centrifugal cleaner is operated. Examples of cyclone-type cleaners connected and used as forward cleaners, in which an accept flow is removed at the base of the cone while a rejects flow is removed from the apex, are shown in Samson et al, U.S. Pat. No. 2,377,524 issued Jun. 5, 1945 and Grundelius et al, U.S. Pat. No. 3,486,619 issued Dec. 30, 1969. In a reverse cleaner system, a cyclone-type cleaner is operated in such a manner that the accept flow is removed from the apex of the cone, while the lighter rejects flow is taken out at the base, as shown in Braun, U.S. Pat. No. 3,912,579 issued Oct. 14, 1975, and in Braun et al, U.S. Pat. No. 3,557,956 issued Jan. 26, 1971.
Forward hydrocyclone cleaning systems are commonly employed to remove heavy particles and contaminants, while hydrocyclone reverse cleaning systems are utilized to remove lightweight contaminants. It is generally known that lower feed consistency leads to better contaminant removal efficiency, in both forward and reverse cleaners. In many installations, the stock preparation equipment includes both forward and reverse cyclone-type cleaners or cleaning systems at discrete locations within the system to produce the particular result for which the cleaning system is efficient. However, there remains a need for combining the forward and reverse cleaners into a single integrated cleaning system, in which the inherent operation characteristics of each system is used to enhance the efficiency of the other.
The prior art also knows so-called "through-flow" cleaners, with a feed port at the base of a cone, and the accepts and rejects outlets at the cone apex. Such a cleaner is described in an article entitled "Through-flow Cleaners Offer Good Efficiency with Low Pressure Drop", Pull & Paper, March 1985 by Terry Bliss. This article also gives a convenient schematic comparison of the through-flow cleaner with the conventional forward and reverse cleaners.
A conventional system for reverse cleaners would include a cascade arrangement, whereby rejects from a first stage are recleaned in a second stage. Secondary accepts join the primary feed, while secondary rejects go to the third stage. Tertiary accepts join the secondary feed and tertiary rejects might leave the system, or go to quaternary stage in the same fashion as described above. Such a system has a high efficiency, but the volume to be pumped through first, second and third stages is very high, and correspondingly the electrical pumping power required is also high. Due to the relatively low reject flow from the entire system, the accepts consistency from the primary cleaner approximately equals the feed consistency, i. e., there is no appreciable thickening or a dilution effect.
A modification of the basic cascade system is shown in FIG. 4 of the Bliss article, giving a somewhat lower power requirement for the system.
Today, conventional through-flow cleaners are used often, because of their low inlet pressure requirement and a low reject rate. The latter results in a small secondary system. Consequently, capital and electrical power requirements are low. Such a system is shown in the Bliss article, FIG. 6. Unfortunately, cleaning efficiency is low, in comparison with that of reverse cleaners, and to compensate, it is today's common practice to use series connected primary through-flow cleaners, i.e., clean the pulp twice. Systems accepts consistency would be about the same as the feed consistency.
Also, combinations of primary reverse cleaners, and secondary through-flow cleaners have been used, as shown in FIG. 5 of the Bliss article. Significantly, secondary accepts are recycled to the primary feed port in conventional cascade fashion. The data in FIG. 5 permit the calculation of the input, consistency, tonnage, and flow rate of the pulp fed to this cleaning system, i.e., the line marked "stock". Accordingly,
feed tonnage=150 T/D PA1 Feed flow rate=maximum 1645 gpm (assuming "0" flow from the white water tank); therefore: PA1 Feed consistency=minimum 1.6% (again assuming "0" flow from the white water tank).
Therefore, very little stock thickening is achieved. Also, some white water flow would normally be permitted, raising the feed stock consistency required even higher.
Obviously, no significant thickening is taking place in this system and the flow rate through the primary reverse cleaner and therefore also the electrical power requirement for that stage is quite high.